AT&T MERLIN LEGEND System Reference Manual page 44

System Overview
With the OPT/loop-start connection, glare is more frequent as the volume of calls
increases. (Glare occurs when a user tries to make an outside call on a
line/trunk at the same time an incoming call is being received on that line/trunk.)
In addition, if system B (the loop-start interface) does not have remote access.
only the stations assigned to the loop-start facility on system B can be accessed
by system A.
Analog Tie-Trunk Connection
In an analog tie-trunk connection, this systems 400EM module is connected to
another system's control unit. If both systems are on the same premises, this
module can be connected directly to the other system if the other system has
similar tie-trunk facilities.
For off-premises connection, the 400EM module can be connected via the
telephone company's facilities to another system.
The analog tie-trunk connection can be administered for two-way traffic or for
one-way traffic (incoming or outgoing). The one-way mode prevents blocked
calls caused by glare.
Digital Tie-Trunk Connection
The digital tie-trunk connection joins the DS1 facility (the 100D module) on the
system to another system's DS1 facility. If the other system is on the same
premises, the connection is back-to-back. To reach a remote system, the DS1
facility connects via a channel service unit (CSU) to the telephone company's
facilities.
Capabilities 1-17